Ragu's Blog

Air Compressor

Compressor

A Gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.

Compressors
are similar to pumps: both increase the pressure on a fluid and both
can transport the fluid through a pipe. As gases are compressible, the
compressor also reduces the volume of a gas. Liquids are relatively
incompressible, so the main action of a pump is to pressurize and
transport liquids.

Types of compressors

Air compressors are mainly classified into two types, based on their construction and operation.

Positive displacement type

Dynamic type

They are as described below in detail:

Centrifugal compressors

Diagonal or mixed-flow compressors

Axial-flow compressors

Reciprocating compressors

Rotary screw compressors

Rotary vane compressors

Scroll compressors

Diaphragm compressors

They are as described below in detail:

Positive displacement type air compressors

Positive
displacement compressors mechanically displace a fixed volume of air
into a reduced volume. They deliver a nearly constant volume,
when operated at a fixed speed; while the discharge pressure is
determined by the system load conditions. The different types of
positive displacement compressors are as follows:

Reciprocating air compressors:

Compressed
air is generated by the to & fro movement of the piston
in the compression chamber (Just like an IC Engine). Each movement
compresses a fixed quantity of free air at a specific
pressure. According to the type of construction, reciprocating
compressors can be further classified as single stage / double stage
and single acting / double acting compressors. The vertical
type air compressors are suitable for applications ranging between
50 - 150 CFM, and the horizontal balance type is most suited for
applications ranging from 200 - 5000 CFM.

Rotary type air Compressors:

Air
is compressed by two rotating, intermeshing rotors (in some cases one
rotor is kept stationery and the other rotates). The action of the
rotary screw / lobe can be compared to a reciprocating
compressor.

Rotary screw compressors
use two meshed rotating positive-displacement helical screws to force
the gas into a smaller space These are usually used for continuous
operation in commercial and industrial applications and may be either
stationary or portable. Their application can be from 3 horsepower (2.2
kW) to over 1,200 horsepower (890 kW) and from low pressure to very
high pressure (>1200 psi or 8.3 MPa).

Rotary vane compressors
consist of a rotor with a number of blades inserted in radial slots in
the rotor. The rotor is mounted offset in a larger housing which can be
circular or a more complex shape. As the rotor turns, blades slide in
and out of the slots keeping contact with the outer wall of the
housing. Thus, a series of decreasing volumes is created by the
rotating blades. Rotary Vane compressors are, with piston compressors
one of the oldest of compressor technologies. With suitable port
connections, the devices may be either a compressor or a vacuum pump.
They can be either stationary or portable, can be single or
multi-staged, and can be driven by electric motors or internal
combustion engines. Dry vane machines are used at relatively low
pressures (e.g., 2 bar) for bulk material movement whilst oil-injected
machines have the necessary volumetric efficiency to achieve pressures
up to about 13 bar in a single stage. A rotary vane compressor is well
suited to electric motor drive and is significantly quieter in
operation than the equivalent piston compressor.

Dynamic type air compressors

Dynamic
compressors mechanically impart a velocity to the air, through the use
of impellers rotating at high speed, in an enclosed housing.
The air is forced into a progressively reduced volume. The
volumetric flow will vary inversely with the differential
pressure across the compressor. The dynamic type of air compressors
are classified into:

Centrifugal air compressors:

The
centrifugal air compressor consists of an impeller, mounted on
a shaft and positioned within a housing, consisting of an inlet
duct, a volute and a diffuser. The impeller rotates at high speed
and imparts a velocity to the air. The diffuser surrounds the
impeller and acts to convert the kinetic energy of the air into
potential energy at a higher-pressure level.

Axial flow air compressors

The
axial flow type air compressor is essentially a large capacity, high
speed machine, with characteristics quite different from the
centrifugal. Each stage consists of two row of blades, one row
rotating and the next row stationery. The rotor blades impart velocity
and pressure to the gas as the motor turns, the velocity being
converted to pressure in the stationery blades.

Roots compressors

This
is really a blower and is generally limited to a pressure of 1 bar in
ingle stage and a pressure of 2.2 bar in two stage combination.

scroll compressor

A
scroll compressor, also known as scroll pump and scroll vacuum pump,
uses two interleaved spiral-like vanes to pump or compress fluids such
as liquids and gases. The vane geometry may be involute, archimedean
spiral, or hybrid curves. They operate more smoothly, quietly, and
reliably than other types of compressors in the lower volume range

Often,
one of the scrolls is fixed, while the other orbits eccentrically
without rotating, thereby trapping and pumping or compressing pockets
of fluid or gas between the scrolls.

Diaphragm compressor

A
diaphragm compressor (also known as a membrane compressor) is a variant
of the conventional reciprocating compressor. The compression of gas
occurs by the movement of a flexible membrane, instead of an intake
element. The back and forth movement of the membrane is driven by a rod
and a crankshaft mechanism. Only the membrane and the compressor box
come in touch with the gas being compressed.

Compressor Cooling System

Cooling
of air compressors merits tremendous improtants since it affects the
energy efficiency. Cooling systems could be either air cooled or water
cooled :

a) Air cooled compressors:
These compressors use fan for forced cooling of the compressors. Due
to the low cooling efficiency, this type of cooling is mostly used for
low capacity compressors having intermittent usage.

b) Water cooled compressors: For heavy duty or continuous applications water cooling system is adopted, as the efficiency of cooling is high.

Location of compressors

Ideally
compressors should be located where it can induct clean, dry and cool
air. One of the major issues related to the compressor
location is the ambient temperature. The compressed air system
exposed to extremely high temperatures can result in higher
specific power consumption, unscheduled shutdowns, increased
maintenance and decrease in lubricant life. Ventilation is equally
important for all compressors regardless of type of cooling.

It
would be interesting to note that for every 4oC reduction in
intake air temperature there is 1% reduction in compressor power
consumption.

At
design stage, it is important to plan for proper ventilation
and access to compressor location. The locations exceeding
temperature of 45oC should be avoided. A free space of about 1 m
around the compressor needs to be provided for maintenance and 1 m for
motor starter access panel.

Addressing
the issue of ventilation at the design stage can help in increasing the
life of the compressor, Lubricating oil and the coolant.

The following points should be considered while locating the compressors.

Low humidity to reduce water entrainment

Adequate ventilation especially for air cooled unit.

Minimum suction piping

Minimum of bends

The quantity of free air delivered by the compressor can be calculated using the following formula.